2. Related Prior Art It is known to produce gears from wrought (i. e., cast and/or forged) metal blanks and from pressed and sintered powder metal preforms.

For many applications, powder metal gears oSer a low cost alternative to wrought gears. Powder metai gears can be formed to near net shape, avoiding costly machining required to produce wrought gears. Howevor, a principe disadvantage associated with powder metal gears made according to conventional practice is that they do not possess the high strength characteristic of wrought gears. The lower strength of powder metal gears is often attributed to low density resulting from traditional manufacturing processes.

Consequently, powder metal gears generally have not been regarded as a practical alternative to wrought gears in applications such as transmission gear components which call for high strength precision gears.

The conventional manner of forming powder metal gears

involves first pressing and sintering a preform to near net shape and therafter roll forming the preform to densify the teeth. The densification is carried out by enmeshing the powder metal preform with a toothed roll forming gear with sufecient force to densify the preform teeth. U. S. Patent 5,711,187 discloses the general arrangement wherein a spur gear powder metal preform is supporte on a spindle for free rotation about a first axis. A spur gear roM forming tool is driven about a second, parallel axis and meshes with the preform to in turn rotate the preform. Greater force is exerted on the driven side of the preform teeth than on the opposite coast side, resulting in corresponding variation in the densiBcation of the teeth. The same difliculties arise in roll forming helical gears.

Further variation can occur as a result of the nature in which the gear teeth of the tool and preform interact. The teeth of the forming tool initial y engage the driven side of the preform teeth and advance radially inwardly, exerting a radially inward wiping or rolling force on the driven side to effect densification. Through firther rotation, the teeth of the tool confront the coast side and advance radially outwardly, exerting a lesser and opposite directed wiping or rolling force on the coast side, leading to the variation in density. Some of the problem can be alleviated by reverse roll forming in the opposite direction, however this operational step adds extra time, cost and complexity to the forming of gears ana may not eliminate the problem entirely.

In roll fomiing involute teeth of powder metal preforms, a trial and error approach is typically used to determine the proper sizing and tooth geometry of the rolling die need to produce the desired tooth profilc, particularly an involute profile, as there is no mathematical basis known using current technology. If it is desired to alter the densification profile of the preform teeth, it is nccessary to change roll forcing dies or alter the starting profile of the preform teeth. As disclosed in the aforementioned'187 patent, it is sometimes necessary to employ a nmber of different fomiing die configurations to generate a desired densified tooth profile.

A similar technique is often used to burnish the teeth of wrought gears with similar difficulties being encountered with respect to burnishing as are encountered with powder metal densification. The solution thus far has been to reverse roll form the wrought gears, but at the cost of adding an extra step of compledty to the gear forming operation.

An object of the present invention is to provide a gear roll fornning apparatus and method which simplifies the roll forming operation of powder metal and wrought gears while mat or altogether eliminating some or all of the disadvantages associated with known traditional processes.

SUMMARY OF TITRE INVENTION AND ADVANTAGES A method of roll forming gear teeth is provided comprising

supporting a gear preform for rotation about a first axis, supporting a fonning roll having a helical worm thread for rotation about a second axis transverse to the first axis, and driving the forming roll and gear preform in rolling engagement with one another with vicient force to displace material ofthe preform and thereby impart a finished gear tooth pattern to the preform.

The roll forming method is particularly useful for densifying the teeth of powder metal gears and burnishing the teeth of wrought gears.

The helical worm thread of the forming roll works radially up and down as well as axially across the teeth of the gear preform on both the drivez and coast sides of the teeth providing high, uniform densification in a single pass, eliminating the need and extra processing step of reverse rot ! forming as is commonly done when employing conventional roll techniques.

The invention also provides an apparatus for roll forming gears comptising a workpiece support spindle for supporting a toothed gear preform for rotation about a first axis, a roll forniing die having a helical worm thread, and a tool spindle mounting the die for driven rotation about a second axis transverse to the first axis. The worm thread of the die meshes with and densifies the teeth of the preform. The apparatus provides the same advantages discussed above with respect to the method of the invention.

The invention further provides a method of modifying a gear

cutting machine tool, nonnally equipped with a gear cutting die having cutting features which remove material from a gear blank, to facilitate roll form densification of the teeth of sintered powder metal gear preforms. The method comprises supporting a sintered powder metal preform having preformed teeth on the machine tool for rotation about a first axis. The cutting die is replaced <BR> <BR> with a roll forming die supporte on the tool about a second axis transverse to<BR> <BR> the first axis of the preform. The roll forming die has a roll forming surface that is substantially free of cutting features that would act to remove preform material The die and preform are driven about their axes in forced meshing engagement, and the relative movements of the die and preform are controlled to density the teeth of the preform. The invention thus provides a simple technique for roll forming powder metal gears by a simple, low cost <BR> <BR> modification to an existing machine tool used to cut gears from wrought gear blanks. A significant avantage that this method offers is that powder metal gears can be densified using similar machine tool programming already employed for cutting wrought gears based on existing mathematical principles <BR> <BR> which, until now, have been reserved for gear cutting operations. Such obviates the current trial and error approach to roll form densifying powder metal gears.

Another significant advantage that this invention offers is that it enables a number of different tooth profiles or densification profiles to be achieved without the need for multiple roll forming dies and die changes.

Tooth shape and densification profiles can be altered by simply controlling the relative drive of the roll forming die and the preform. Without changing dies, an involute helical tooth probe can be achieved, as well as crowned powder metal gear teeth. It will be appreciated that significant cost and time savings is recognized by the subject densuying processes.

BRIEF THEDRAWINGSOF These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appende drawings, wherein: Figure 1 is a schematic perspective view of a gear roll forming apparatus constructed according to one embodiment of the invention; Figure 2 is an enlarged fragmentary sectional view taken generally along lines 2-2 of Figure 1; Figure 3 is an enlarged fragmentary sectional view taken generally along lines 3-3 of Figure 1; Figure 4 is a further enlarged sectional view illustrating details of the interaction between the forming tool and gear preform ; Figure 5 is a schematic perspective view of a gear roll fonning apparatus for forming helical gear teeth according to a second embodiment of the invention; and

Figure 6 is an enlarged fragmentary plan view of powder metal gear that has been roll form densified according to the invention having a crowned tooth profile.

DETAILED DESCRIPTION A gear roll forming apparatus constructed according to a presently preferred embodiment of the invention is indicated generally at 10 in Figure 1 and comprises a workpiece spindle 12 having a rotation axis A for mounting a gear workpiece or preform 14 for rotation about the axis A. The apparatus further includes a tool spindle 16 adjacent the spindle 12 having a rotation axis B transverse to the rotation axis A A roll forming die 18 is mounted on the spindle 16 for rotation about the axis B in operative working <BR> relation relative to the preform 14. The axis B is skewed relative to the axis A<BR> (that is, axes A and B are nonparallel and noninterse «) As shown best in Figures 1 and 2, the forming die 18 comprises a generally cylindrical body 20 having peripheral roll forcing teeth in the form of a helical worm thread 22. The thread 22 has opposed leading and trailing fbrmng faces 24,26 (with respect to the direction of rotation) extending helically about the cylindrical body 20 of the die 18 continuously between opposite axial ends 28,30 of the die 18. The forming thread 22 is free of any cutting teeth or features that would act to cut, shear or remove martial from

the preform 14 in the sense of a gear cutting operation. Rather, the objective of the forming thread 20 is to displace the martial of the preform in order to <BR> <BR> dent or burnish the preform as will be discussed betow. The forming thread 22 is preferably a single start thread, although multiple start threads are contemplated by the invention to the extent they would perform the same or simi ! ar function of densifying or burnishing the preform in the same or similar way. The number of worm thrcads, Nw in a die 18 having a single start thread is one, whereas Nwis greater than one for dies having multiple start threads or3).(e.g.,2 The gear preform 14 is initially formed in the general shape of a gear having a number, N, of gear teeth 32 extending radially from a prefonn body 34. Each tooth 32 has opposite leading 36 and trailing 38 faces with respect to the direction of rotation of the preform during roll forming, and each tooth 32 is separated circumferentia ! ly from an adjacent tooth by a space 40.

The teeth 32 are formed initially slightly oversized relative to a final finished dimension after roll forming to account for the slight dimensional changes that occur as a result of the roll forming operation. Any of a number of gear teeth profiles may be formed, including spur, helical1 involute and/or crowned.

The gear preform 14 may be fabricated cither of power metal which has been compacted and sintered, or may comprise a wrought gear preform formed by such conventional techniques as casting, forging, octruding,

hobbing, shaping, or the like. The selection of material for the powder metal preform is dependent in part on the particular end application. In an application such a transmission gear component for example, a high strength steel alloy would be suitable. The same material selection criteria would apply with respect to metallic wrought gears.

In the roll forming operating according to a presently preferred embodiment of the invention, the roll forming die 18 is mounted on the tool spindle 16 and the preform 14 is mounted on the workpiece spindle 12 so that their axes A, B lie transverse to one another with the outer periphery of the forming tool 19 adjacent the outer periphery of the preform 14.

The preform 14 and forcing roll 18 are initially spaced from one another out of meshing engagement to accommodate tl : e loading of the preform 14 onto the workpiece spindle 12. Either the workpiece spindle 12, tool spindle 16, or both are supported for lateral movement toward one another in the direction of arrow C (Figure 1) in order to bring the forming roll 18 and preform 14 into forced meshing engagementwith one another. The spindle 16 is coupled to a drive motor 42 which is operative to positively drive the spindle 16 and hence the fonTiing roll 18 about the axis B in the direction of arrow D <BR> <BR> <BR> <BR> <BR> <BR> (Figure 1) or in the opposite direction if desired, at an angular velocity ou. The<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> rotation of the forming roll 18 in the direction of arrow D imparts corresponding rotational force to the preform 14 urging it to rotate in the

direction of arrow E (Figure 1) due to engagement of the enmeshed thread 22 of the die 18 with the teeth 32 of the preform 14. In the preferred embodiment, the preform 14 is also positively driven by an associated motor 44 coupled to the workpiece spindle 12 to enable independent control of the relative rotation and angular velocity mu of the preform 14 for purposes which will be discussed below.

Figures 2-4 illustrate the interaction that takes place between the worm thread 22 and the preform gear teeth 32 during roll forming that accounts for uniform, high densification of the teeth of a powder metal preform, or uniform burnishing of a wrought preform. Referring initially to Figures 2 and 4, as the worm thread 22 enters the space 40 between adjacent teeth of the preform 14, identified as 32a and 32b in Figure 4, the leading wall 26 of the worm thread 22 initially contacts the trailing face 38 of the leading tooth 32a with respect to the direction of rotation E of the preform 14. As the die 18 rotates, the leading face 26 of the worm thread 22 progresses radially down the trailing face 38 of the tooth 32a and exerts a radially downward wiping action-In addition to the radial progression of the worm thread 22, the worm thread 22 traverses axially across the teeth 32 from one axial side 46 to the opposite axial side 48 of the gear preform 14 in the direction of arrow E (see Figure 3), thereby imparting an axial wiping force in addition to the radial wiping force to the preform 14.

Figure 4 illustrates, in stepped progression, the movement of the worm thread 22 between the adjacent teeth 32a, 32b. Continued rotation of the die 18 brings the trailing face 24 of the worm thread 22 into engagement with the leading face 36 of the trailing tooth 32b across the space 40, imparting the same radial and axial wiping force on the tooth face 36, but with the radial component being directed up (or radially outwardly) of the tooth 32b in the opposite direction. The radial and axia ! wiping contact of the worm thread acts on the oversized teeth 32 to displace the material of the preform 14 at the contacting surfaces of the teeth 32 without removing material to any significant degré (as in the sense of gear cutting operations). In the case of a powder metal preform, the resultant displacement of the preform material acts to densify and harden the teeth 32 to a predetermined depth as illustrated by the stipled regions of Figures 2 and 4. FuI theoretical density is achievable to various depths, with the degree and depth of densincation being dependent upon the particular application.

The same principles described above apply when roll forming a wrought preform, with the stipled region 50 representing the aSected burnished zone resulting from the displacement of the preform material by the worm thread 22.

Positively driving the die IS and preform 14 independently of nne another enables adjustment in the directional loading applied by the thread

22 of die 18 on the teeth 32 of the preform 14. For exampte, stowing the <BR> <BR> <BR> <BR> <BR> rotational drive of the preform 14 relative to the normal rotation imparted by the die 18 acts to introduce resistance to the rotation of the preform 14 and thus increases the loading applied by the worm thread 22 on the trailing face 38 of the gear teeth 32. Increasing the rotational drive of the preform 14 correspondingly increases the load applied by the thread 22 on the leading face 36 of the gear teeth 32. Particular tooth forms can thus be generated by controiung the relative drive of the die 18 and preform 14. For producing an involute tooth form as illustrated in Figures 2 and 4, for example, the angular cot N velocities (ow, me are controlled such that the drive ratio =Adjusting wu Nw the relative drive of the die 18 and preform 14 during a given gear rolling operation can be used to achieve other tooth forms and to produce uniform densification of the teeth or to densify a particular selected region of the teeth 32 more or less than other regions.

Figure 5 shows an alternative arrangement of a roll forming apparatus 110 according to the invention, wherein like reference numerals are used to represent like features but are offset by 100. In this case, the preform 114 has a helical tooth form 132 that may have an involute profile representative of that shown in Figures 2 and 4 with respect to the preform 114. The same general set up is employed as that shown in Figure 1. except that the angular relationship between the axes A, B is adjusted to accommodate

the helical tooth form of the preform 114. In Figure 1, the teeth 32 are taraud across the preform 14 with the axis A and the angle between the axes A, B is set at 90°. In Figure 5, the helical teeth 132 are set at an angle to the axis A of the preform 114, and thus the angle between the axes A, B is adusted to account for the helical tooth form such that the thread 122 of the die 118 interacts with the teeth 132 of the preform as described previously with respect to the first embodiment of Figures 1-4.

Figure 6 shows still a further alternative tooth form that may be generated by controlling the relative movements and drive of the die 18 and preform 14 including overdriving and underdriving the preform 14 relative to the movement imparted by the worm 12 during predetermined times of the roll forming cycle. Figure 6 is a top or plan view looking down on the tip of a gear tooth 32 and it can be seen that the tooth 32 has a crowned profile such that it is narrow at its ends and is enlarged in the puddle Such a crowned tooth <BR> <BR> <BR> <BR> <BR> profile is not new per se with respect to the manufacture of wrought gears (i. e., gear cutting and shaping operations), but is not heretofore been known to be produced on a surface densified powder metal gear using a single rolling die through controlled relative movement and drive of both the die and gear preform as according to the present invention. The crowned tooth profile can be provided on helical gear teeth, spur gear teeth, etc.

According to another aspect of the invention, a convenient and

cost-effective method of roll form densifying powder metal gear preforms of the invention is by modifying existing machine tools of the type that are traditionally used to cut geat teeth of wrought gears (e. g.. hobbing and shaping machine tools) in such manner as to enable such machine tools to roll form densify powder metal gear preforms.

The traditional gear cutting machine tools have a cutter head formed with a series of cutting surfaces or teeth which act to remove material from a wrought gear blank-to forrn the gear teeth having any of a number of shapes and profiles, including spur gear teeth, helical teeth, involute profiles and crowned tooth profiles discussed above as well as these and others which are known by those skilled in the art of the manufacture and operation of gears.

In a traditional gear hobbulg machine used for cutting gear teeth of wrought gear blanks, the cutter or hob is in the form of a worm having a helical thread, across which are formed deep grooves revealing a number of cutting edges or surfaces of the thread which remove material from the gear blank to form the teeth of the gear. The formation of a particular gear tooth pattern or profile is controlled by the relative positioning and movements of the hob cutter and gear blank according to pre-programmed cutting operations. Similarly, gear cutters and shapers employ a rotating head with cutting edges or surfaces with act to remove material from the gear blank to forum gear teeth.

According to the invention, such machines are modified by

mounting the powder metal gear form on the spindle which would normally support a wrought gear blank, and replacing the cutting head with a roll forming die 18 according to the invention having a similar form as that of the cutting head but lacking the cutting surfaces. For example, Figures 1-6 schematically illustrate a modified gear hobbing machine having the roll fornsing die 18, 118 with a helical thread 22,122 which extends continuously and lacks the deep grooves which a hob cutter has to provide cutting surfaces Such cutting surfaces are purposely lacking from the roll forming die 18,118 since the objective is to displace and density the prefor » z rnaterial rather than cut and remove the material. Advantageously, the pro~8 normally used to operate the machine tool to cut a particular gear tool form can be used to operate the roll forming die 18, 118 to densify the powder metal preforms 14 according to the invention, with adjustment as necessary in the relative drive and movements of the roll forming die 18, 118 and powder metal preform 14, 114, to achieve the desired densification profile of the teeth as previously described.

Acxordingly, the invention further contemplates a roll fanning die having the general shape of a cutting head for gear cutting operations, but lacking the cutting edges or surfaces.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may bc practiced otherwise than as Theinventiondescribed. is by theclaims